Methane, the primary component of natural gas, is perhaps the most desirable fossil fuel. It is thermodynamically stable, has very high energy content, and is readily transportable with existing pipeline infrastructure. It is currently used in almost all energy applications, even as a transportation fuel. Methane is used to produce most of the world's ammonia as well as many other chemicals. In many parts of the world such as the U.S. natural gas production has not kept up with increased demand for this fungible energy source, which has a smaller carbon footprint than other fossil fuel sources. Methane is an important energy source used for power production, building heating, hot water, and cooking. Methane is also growing as a transportation fuel. In comparison to coal or petroleum, methane releases significantly smaller amounts of carbon dioxide per unit of energy produced.
Renewable sources for methane are increasingly important. Microbial conversion of carbonaceous feedstocks to methane by anaerobic bacteria and archaea is well known and used in many operations. Biogenic methane (“biogas”) production is used in municipal wastewater treatment to convert sewage and activated sludge to methane to recover some of the energy and reduce the mass of waste sludge that has to be disposed. Methanogenesis is also used to treat waste from food, agricultural and chemical process industries to recover carbon and energy and reduce waste discharge loads and costs. In animal feedlots that are being increasingly used for poultry, swine and beef production, the wastes are digested to reduce discharge loads, recover some energy and reduce treatment costs. Numerous small scale digesters are used to treat human and other animal waste for the same reasons, especially in the rural areas of the less developed countries. In municipal solid waste landfills, biological methane production occurs after a period of time and now the more recent landfills are being designed and engineered to enhance biological methane production and recover energy values.
The main carbonaceous feedstocks that typically are utilized for biogas production include waste materials comprising one or more of (a) biopolymers, e.g., cellulose, hemicellulose, lignin, pectins, and the like; (b) fats and oils; (c) proteins; and (d) other soluble and semi-soluble organics. Based on numerous studies it is generally accepted that a consortia of anaerobic microorganisms e.g., hydrolytic acidogens, syntrophic acetogens, and methanogens, work together via highly self-regulated mechanisms to bring about this bioconversion.
Methanogenic microbial consortia naturally produce methane from a variety of carbonaceous sources. The energy content of the coal and other hydrocarbon materials is conserved in the produced methane. To balance the redox equation, CO2 (as bicarbonate) is produced concomitantly, e.g., according to the formula: 2CH (e.g., coal)+2H2O→CH4 (for energy)+CO2 (as HCO3−). This is a natural process that occurs in coal beds where coal bed methane (CBM) is produced, generally by the action of a consortium of anaerobic microbes typically in a biofilm around the coal surface. The same process occurs in any anaerobic fermentation of carbonaceous materials utilizing methanogenic microorganisms. A typical methane digester converting sewage or other carbonaceous feedstocks to methane produces a gas that is typically 50 to 70% methane, with the remaining 30 to 50% being predominantly CO2, with trace amounts of other gases, including hydrogen sulfide. In July of 2014, the Untied States Environmental Protection agency (USEPA) ruled that biogas qualifies as a cellulosic biofuel.
There is an ongoing need for new methods of biogenic methane production, particularly methods that provide higher concentrations of methane and/or lower concentrations of undesirable gas contaminants such as carbon dioxide and hydrogen sulfide in the produced biogas. In addition, development of new sources for cellulosic biofuels and new energy sources are in the public interest. The methods described herein address these needs and interests.